Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
  • H04L1/0056—Systems characterized by the type of code used
  • H04L1/0059—Convolutional codes
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
  • H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
  • H03M13/39—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes
  • H03M13/3955—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using a trellis with a reduced state space complexity, e.g. M-algorithm or T-algorithm
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
  • H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
  • H03M13/39—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes
  • H03M13/41—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using the Viterbi algorithm or Viterbi processors
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
  • H03M13/37—Decoding methods or techniques, not specific to the particular type of coding provided for in groups H03M13/03 - H03M13/35
  • H03M13/39—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes
  • H03M13/41—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using the Viterbi algorithm or Viterbi processors
  • H03M13/4138—Sequence estimation, i.e. using statistical methods for the reconstruction of the original codes using the Viterbi algorithm or Viterbi processors soft-output Viterbi algorithm based decoding, i.e. Viterbi decoding with weighted decisions
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
  • H04L1/0045—Arrangements at the receiver end
  • H04L1/0054—Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits

Definitions

• a transmission link between a transmitter and a receiver will suffer from a number of impairments. Two such effects are thermal noise and multipath fading.
• ISI Intersymbol Interference
• Some digital communication systems also introduce ISI by design. This is the case in the E- GPRS system, where the modulation pulse shape used to improve the spectral efficiency of the transmitted signal generates ISI.
• the modulation pulse shape used to improve the spectral efficiency of the transmitted signal generates ISI.
• 3GPP TS 05.04 V8.4.0 2001-11
• Technical Specification 3rd Generation Partnership Project Technical Specification Group GSM/EDGE Radio Access Network
• Digital cellular telecommunications system Phase 2+
• Modulation Modulation.
• TDMA Time-Division Multiple Access
• FEC Forward Error Correction
• An FEC coding operation introduces redundancy to a transmitted signal and this redundancy can then be used at a receiver to improve the accuracy of estimates of the transmitted data that are generated by the receiver.
• FEC coding in a transmitted signal it is important that such a signal is demodulated by a receiver in a format which can be best interpreted by an FEC decoding process within the receiver.
• a number of such receivers have been proposed in the past. See, for example, Optimal decoding of linear codes for minimizing symbol error rate', L. Bahl, J. Cocke, F. Jelinek, J.
• the error protection coding unit 101 generates from the block ⁇ M t ⁇ Ae /, r) a
• D (where D > K) information bits, d k e ⁇ 3 l ⁇ .
• Each set of M information bits A k is modulated onto the complex plane using a modulation scheme M that maps sets of M bits onto the complex plane. This mapping is performed by a modulation unit 102.
• the modulation M can be expressed as:
• a set of M information bits can be uniquely identified with a single decimal number / ⁇ ⁇ z ⁇ 2 M ⁇ '
• This equation maps the Af binary values in a set ⁇ , to a unique value in the set
• the C modulated symbols representing a transmission block are transmitted over the air and in the process are distorted by the propagation channel 103.
• the samples ⁇ s k ⁇ . at the input of a receiver can be expressed as:
• any filtering performed by either the transmitter and/or the receiver can be incorporated in the propagation channel model.
• the mappings F and S used to model the propagation channel can be time varying. However, to simplify the notations, it is assumed in this document that these mappings do not depend on time. Note, however, that the approach described here is also applicable to time-varying channels.
• propagation channel mappings F and S can be modelled as linear filtering operations:
• the channel mapping can be described with j ust a set of filter coefficients ⁇ fy ⁇ /s(0 L _ j . •
• the receiver In order to recover the transmitted symbols c & the receiver will need to know the propagation channel mapping. However, the receiver will usually not have prior knowledge of the propagation channel conditions. It may nevertheless be possible for the receiver to generate estimates of the channel coefficients ⁇ h, ⁇ , 0 L 1 , which can be used in place of the true values.
• a transmitted signal will incorporate a pattern, referred to as training sequence, which is known to the receiver and the receiver can use this training sequence to generate an estimate of the propagation channel conditions.
• the signal is, ⁇ . . is corrupted by additive noise, introduced at process 104, such that a block of symbols from which the receiver tries to recover transmitted information can be expressed as:
• r k s k +n k (* e ⁇ l,...,C ⁇ )
• the aim of a receiver is to generate the sequence ⁇ u ⁇ , ,, _> which maximises the set of A-
• a full maximum A-Poste ⁇ ori receiver will estimate a sequence of transmitted bits [u k ] . by taking into account the error correction coding and the modulation scheme at the same time.
• Such a receiver will perform well and allow a good throughput for a transmission link.
• the implementation complexity of such a receiver is almost always prohibitive.
• sub-optimum solutions are usually preferred.
• Practical solutions usually separate the demodulation from the error correction decoding, as in Figure 2.
• the sequence of received symbols ⁇ r, ⁇ , , mecanic. is used by unit 201 to calculate estimates of the transmitted coded bits ( ⁇ j 46/ , D y
• the error correction decoding unit 202 uses the symbols
• the algorithms used for the error correction decoding 202 can be divided into two broad classes depending on the type of inputs being used.
• the first class corresponds to receivers which use hard decision decoding. These decoding techniques use only the estimates of the coded bits ⁇ d ⁇ in order to produce estimates ⁇ , ,, _ of the uncoded bits.
• second class corresponds to receivers which use soft decision decoding.
• These decoding technique rely not only on the sequence of estimated bits I d k but also make use of
• I He(I, ,D) decoding receivers usually perform better than hard decision decoding receivers.
• the information input to the error correction decoding unit 202 is the Log-Likelihood Ratio (LLR), which for the coded bit d k is expressed as:
• estimates of the coded bits can be generated from these LLR using the following rule:
• the invention provides apparatus for manipulating a received signal which corresponds to a transmitted signal comprising a series of bits, the apparatus comprising means for processing the received signal using a Viterbi algorithm to produce candidate metrics for states arranged in a trellis and to make selections from amongst the candidate metrics to provide metrics for said states, means for producing an initial soft decision for a bit of said series from said candidate metrics, means for producing a hard decision for said bit from a metric selection corresponding to reception of said bit and means for modifying the initial soft decision in a manner dependent upon the hard decision to produce an enhanced soft decision for said bit.
• invention also consists in a method of manipulating a received signal which corresponds to a transmitted signal comprising a constitus of bits, the method comprising a step of processing the received signal using a Viterbi algorithm to produce candidate metrics for states arranged in a trellis and to make selections from amongst the candidate metrics to provide metrics for said states, a step of producing an initial soft decision for a bit of said series from said candidate metrics, a step of producing a hard decision for said bit from a metric selection corresponding to reception of said bit and a step of modifying the initial soft decision in a manner dependent upon the hard decision to produce an enhanced soft decision for said bit.
• the initial soft decision is produced as a log likelihood ratio from candidate metrics.
• the hard decision is produced by storing a decision history of metric selections that are made in calculating metrics through the trellis and then tracing back through the decision history to produce the hard decision.
• the trace-back operation may be initiated after processing only a portion of the trellis.
• metrics are produced for the trellis states using a reduced-state technique in which metric selections made in moving through the trellis are treated as symbol decisions for calculating metrics further along the trellis and one of the metric selections is selected to provide the hard decision.
• the metric selection may indicate a modulation symbol that requires translation to recover the hard decision.
• the value of the enhanced soft decision is selected from a group of values conditional on the state of the hard decision. In certain embodiments, the value of the enhanced soft decision is selected from a group of values conditional on the polarity of the initial soft decision.
• the invention provides apparatus for manipulating a received signal using a Viterbi algorithm to produce metrics for states in a trellis, wherein the received signal corresponds to a transmitted signal comprising a series of bits and the apparatus comprises means for performing an update of the metrics corresponding to reception of a bit by calculating candidate metrics for all the states, means for calculating a function of a plurality of the candidate metrics that correspond to a decision that the bit is a logical one, means for calculating a function of a plurality of the candidate metrics that correspond to a decision that the bit is a logical zero and means for calculating the difference of the values of the two functions.
• the invention also consists in a method of manipulating a received signal using a Viterbi algorithm to produce metrics for states in a trellis, wherein the received signal corresponds to a transmitted signal comprising a series of bits and the method comprises a step of performing an update of the metrics corresponding to reception of a bit by calculating candidate metrics for all the states, a step of calculating a function of a plurality of the candidate metrics that correspond to a decision that the bit is a logical one, a step of calculating a function of a plurality of the candidate metrics that correspond to a decision that the bit is a logical zero and a step of calculating the difference of the values of the two functions.
• Received bits used in progressively calculating metrics through the trellis may arrive as part of a modulated symbol conveying more than one bit of the transmitted signal. It may be the case that modulated symbols rather than received bits per se are used to update trellis metrics.
• routines according to the present invention for manipulating metrics and soft decisions can be implemented in hardware or as software for execution by suitable data processing hardware. These routines can implemented within, for example, a base station of a wireless communications network or a telephone for use in such a network.
• the manipulation of the received signal by the invention may be for the purpose of demodulation or decoding of the signal.
• the trellis states may by defined by the possible permutations of symbols in the channel memory.
• the trellis states may be defined by the possible permutations of symbols in a memory arrangement forming part of a convolutional encoder involved in the production of the received signal.
• Figure 1 illustrates a model of a communications link
• Figure 2 illustrates a genre of receiver architectures
• Figure 3 illustrates a full-trellis demodulator scheme according to an embodiment of the invention
• Figure 4 illustrates a reduced-state trellis demodulator scheme according to an embodiment of the invention.
• Figure 5 illustrates a receiver architecture suitable for hosting a demodulator scheme according to the invention.
• the proposed receiver architecture also has the general structure shown in Figure 2.
• the demodulation unit 201 uses Viterbi processing to produce a stream of soft decisions, in the form of LLRs or estimates of the LLRs, for use by the error correction decoding unit 202.
• the different computations involved in the Viterbi processing will not be detailed and only those computations that facilitate the description of this embodiment of the invention will now be presented. For a more detailed description of the computations involved in the Viterbi processing used here, see, for example, 1 TCM on frequency-selective fading channels: a comparison of soft-output probabilistic equalizers', P. Hoeher, GLOBECOM '90, December 1990.
• the implementation of the Viterbi processing used in the demodulation unit 201 will first be described for the case of a full trellis (i.e. all the hypotheses for the transmitted bits associated with the different channel taps are taken into account); the case of a reduced-state trellis will be described later.
• the number of states in the trellis used to perform the Viterbi decoding, denoted S is equal to 2 Mxl .
• any given state in the trellis has 2 M "merging states" leading into it.
• H is a function that maps a value from the set ⁇ , ..., S — l ⁇ to a value in a set of L-I complex numbers.
• ⁇ ⁇ m ⁇ 5'-l;O ⁇ / ⁇ 2 A/ 1 J is the index of the state leading into the state with index m in the path that corresponds to a hypothesis transmitted complex symbol c equal to M(D- (O) .
• the branch metrics are calculated using the following equation:
• the hypothesis samples c k _ u used in the above branch metric calculation are taken from the states p(m,i) and m that the branch metric interconnects.
• branch metric described in the above equation can be calculated in other ways. It is possible to modify the computation of the branch metric, based on, for example, knowledge of, or an estimation of, the statistical distribution of noise affecting the transmitted block.
• the LLR for the set of transmitted bits are then calculated using the following equation:
• the LLR can be improved by grouping first all the metrics corresponding to a decision 1, then all the metrics corresponding to a decision 0 and calculating the difference. This is shown in the following modified equation:
• the decisions made in the calculation of the state metrics needed to produce the initial LLRs are stored in a decision history.
• the information stored in the decision history is the modulated symbol i to which the decision corresponds, as given by the following equation:
• the initiaI LLRs are generated in increasing order of k in a forward computation phase, whereas, during the trace-back processing, the estimated transmitted bits ⁇ d Mx ⁇ k _ My d Mx ⁇ k _ M)+v ...J Mx ⁇ k _ M)+M ⁇ aie generated in decreasing order of k.
• the trace-back processing can be performed starting from this known state.
• the trace-back can be performed from the state which the state metrics indicate as being the most likely.
• the trace-back processing can be performed at any point during the aforementioned forward computation stage.
• the selection of the point at which trace-back processing is performed allows memory requirements to be balanced against computational complexity.
• the enhanced LLRs have improved accuracy. Since the bit decisions ⁇ Mx i k - L+ D +b are g enera ted from the processing of the whole burst, the enhanced LLR values make use of the redundant information embedded in the received signal that the initial LLR value do not employ. Moreover, the increase in complexity and memory requirements caused by the trace-back and LLR modification processes is very limited.
• the enhanced LLRs are generated using the following rule:
• the enhanced LLRs are generated using the following rule:
• the association rule between the estimated bit d Mx ⁇ k _ L+ ⁇ +b and the sign of the enhanced LLRs can be changed to match the decision convention used by the receiver.
• FIG. 3 presents the different processing stages that are involved in the generation of enhanced LLRs from initial LLRs.
• the state metrics update unit 301 calculates the state metrics ⁇ m (k + l) using the maxima of the candidate metrics of the braches that lead into the states. As part of these state metrics update computations, the various candidate metrics ⁇ k+ ⁇ (/) and the different best candidate metric indices T m (h+ ⁇ ) are generated as described earlier.
• the candidate metrics ⁇ k+x (/) are used by unit 302 in order to derive the initial LLRs ⁇ /x f /t - L+iUi • Once calculated, the initial LLRs are stored in an LLR memory 304.
• the branch metric indices F n , (k+ ⁇ ) are stored in a path history decision memory 303. Then, when the trace-back point is reached (for example at the end of the transmission block), the initial LLRs are, using one of the rules described above, combined in unit 305 with the decisions d M , k _ L+ ⁇ )+h derived from the trace-back processing in order to derive the enhanced
• units 301 to 305 are implemented as software elements for execution by a microprocessor with the functions of memories 303 and 304 being served by a single memory associated with the microprocessor.
• the demodulation stage of the proposed receiver architecture can employ reduced-state, rather than full-trellis, Viterbi processing.
• L long propagation channels
• M high-order modulation schemes
• the implementation complexity of the full trellis approach is often prohibitive.
• the total number of states, even for channels with relatively low delay spreads, is far too large to be efficiently implemented in a mobile receiver.
• Different approaches have been proposed in the past in order to reduce the number of states to be processed in the receiver.
• DDFSE Delayed Decision Feedback Sequence Equaliser
• RSSE Reduced State Sequence Estimator
• a DDFSE architecture will now be described in which bit decisions d M , k _ u ⁇ , b are used to produce enhanced LLRs. It will, however, be understood that the principles of the schemes described in this document for modifying LLRs using hard decisions about transmitted bits can also be applied to receivers using different reduced-state Viterbi processing architectures, such as the RSSE.
• the number of states for which trellis processing is performed is reduced from 2 Mx ⁇ L ⁇ ⁇ to 2 ⁇ r ' . To reflect this change in the number of states, the branch metric computations need to be modified as follows:
• the ⁇ m (k + 1) are compared for all m to deduce which state is most likely to be correct.
• Inherent in that state is a set of hard decision symbols c k _ u (m) where u e j ,..., L j - + L r - I j .
• Figure 4 presents a receiver architecture for reduced-state processing that can be used to implement the DDFSE architecture described above.
• the state metrics ⁇ m (k + l) and the sets of past decision symbols c k _ u (w) that are attached to the states are updated by unit 401.
• the different candidate metrics ⁇ M (/) are generated.
• These candidate metrics are used by unit 402 in order to derive the initial LLRs ⁇ . ,, , v . These initial values are then combined in unit 403 with the bit decisions
• the proposed receiver architecture for the reduced-state approach need not require any memory storage for the initial LLRs or the best candidate metric indices T 111 (k + l) .
• the proposed receiver architecture only requires storage for the state metrics (as is the case for any Viterbi processing) and for the past decision symbols c k ⁇ (jn). For each state, only the latest
• FIG. 5 illustrates a generic structure that can represent a mobile telephone, base station or similar receiver in which the invention can be implemented.
• the receiver 501 comprises an antenna 502 for acquiring wireless signals, an RF section 503, an analogue-to-digital conversion (ADC) section 504, a data processor 505 and a memory 506.
• ADC analogue-to-digital conversion
• the receiver will contain many other elements but only those necessary for an explanation of the implementation of the invention have been shown.
• Signals received at the antenna 502 are down-converted in frequency and amplified at RF section 503.
• the signals are then converted into digital signals by ADC section 504 and passed to processor 505.
• the processor 505 performs the operations necessary to extract and utilise the information payload of the acquired signals, relying on the memory 506 to provide storage of signal values and other data as required.
• the processor 505 undertakes the routines described above according to the present invention for manipulating metrics and soft decisions, such as those outlined in Figures 3 and 4.

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